This invention relates to the recording of video and information signals, such as PCM audio information signals, in time division multiplexed form and, more particularly, to such recording of high definition television signals (HDTV signals) which minimizes the possibility of crosstalk interference in the event that the non-video information signals are recorded at a later time (known as "after-recording").
Video recording systems are known to record and reproduce a video signal and a pulse code modulated (PCM) audio signal. In one such system, the PCM audio signal may be modulated for recording purposes in a section of the frequency band that is separate and apart from the spectral band occupied by the video signal. This permits the PCM audio signals and the video signals to be frequency multiplexed and recorded in a common track on, for example, magnetic tape.
In another recording system, the magnetic tape is wrapped about a rotary head drum to define a wrap angle that extends for more than 180.degree.; and the video signal is recorded in a track length corresponding to about 180.degree. while the PCM audio signal is recorded in the remaining portion of the track that extends beyond the 180.degree. length. In this latter system, the PCM audio signal is recorded first followed by the video signal, resulting in a track length corresponding to an angular extent of about 221.degree.. That is, the rotary head which is used to record the PCM audio signal and the video signal rotates in contact with the magnetic tape over an angle of about 221.degree..
Often, it is desirable to record a video signal first and then, at some later time, record the audio signal, such as in a dubbing mode. This after-recording (as opposed to simultaneous recording) of the audio signal is quite difficult and complex to execute in the system wherein the video signal and the PCM audio signal are frequency multiplexed.
While after-recording of a PCM audio signal in a recording system of the type wherein the PCM audio signal and the video signal are time division multiplexed for recording in separate lengths of a record track is simpler to carry out, such after-recording, particularly when used for dubbing purposes, usually is performed in conjunction with video signal playback. That is, after a head scans a record track to reproduce video signals therefrom and then is changed over to a recording mode, when it reaches the discrete section of the track in which the PCM audio signal is to be recorded. However, and as will be described in greater detail below, when plural recording heads are used (as is common), at the time that one head is positioned for recording the PCM audio signal, the other head still is scanning the video portion of a track and, thus, the recording of the PCM audio signal overlaps in time with the reproduction of the video signal. Even though the same head is not used for simultaneous recording and playback, nevertheless, this overlap tends to reduce the quality of the reproduced video signal.
FIG. 1 illustrates the configuration of a magnetic tape wrapped about a rotary head drum over an angular extent of more than 180.degree., wherein the PCM audio signal and the video signal are recorded in a common track, with the PCM audio signal being recorded ahead, or upstream, of the video signal. If two heads spaced apart by 180.degree. are mounted on the rotary head drum, it is seen that one head will be in position to reproduce the video signal when the other head is in position to record the PCM audio signal. To reduce the possible loss in quality of the reproduced video signal caused by this overlap condition, separate recording and playback amplifiers and a change-over switch normally are provided. However, because of inherent electronic and mechanical time delays and the desire to reproduce an entire video signal from a complete track, it is virtually impossible to reduce the overlap condition to zero. Moreover, such recording/playback circuitry is rather complicated and this too detracts from the reliability of the video tape recorder and tends to degrade the overall quality of its operation.
In another proposal in which the video and PCM audio signals are recorded in the same track, multiple tracks are recorded simultaneously by a number of record/playback heads. Here, the combined video and PCM audio signals are distributed to a plurality of channels, each of which is recorded by a respective head. When, for example, two tracks, or channels, are recorded simultaneously, the two heads used therefor are displaced from each other in the track-scanning direction by an amount corresponding to an angle .THETA.. As shown in FIG. 2, the leading head is displaced from the trailing head by .THETA., resulting in a corresponding longitudinal displacement of the respective tracks due to the fact that the leading and trailing heads record signals simultaneously.
It is noted that, in the multi-track arrangement shown in FIG. 2, the trailing head scans the video signal recorded in one track simultaneously with the scanning by the leading head of the PCM audio signal recorded in the adjacent track. Thus, in the after-recording mode, during the angular extent .alpha., the trailing head plays back the video signal while the leading head records the PCM audio signal. Hence, the aforementioned overlap problem is present. Furthermore, because the PCM audio signal is adjacent the video signal during the angular extent .alpha., a crosstalk component tends to occur during signal playback because these adjacent signals are not correlated with each other.
To reduce the crosstalk phenomenon, the signal supplied for recording by the trailing head is delayed by an amount .tau. corresponding to this displacement angle .THETA.. As a result, the simultaneous recording of, for example, two tracks appears as shown in FIG. 3, wherein these adjacent tracks are aligned or matched with each other. Both heads record the PCM audio signals and the video signals at the same time, and the presence of crosstalk due to the simultaneous scanning by the respective heads of non-correlated signals is avoided. However, the recording of matched, adjacent tracks nevertheless suffers from the drawback described in conjunction with FIGS. 4A-4C.
FIG. 4A illustrates the signals played back from successive scans of the tracks recorded on a magnetic tape. Let it be assumed, for simplification, that each track is formed with a leading section in which the PCM audio signal is recorded, followed by the video signal. Assuming that two tracks are recorded simultaneously by two heads angularly offset from each other by .THETA., and further assuming that the signal supplied to the trailing head is delayed by .tau. to compensate for this angular offset, FIGS. 4A and 4C illustrate the signals supplied for recording. The resultant tracks are matched, as shown in FIG. 3, and when the signals recorded therein are reproduced, to account for the time delay .tau., the signals reproduced by the leading head are delayed by this same amount .tau.. Hence, FIG. 4B illustrates the signals reproduced by the trailing head and FIG. 4C illustrates the delayed signals reproduced by the leading head. It is appreciated that this delay .tau. imparted to the signals reproduced by the leading head is needed to restore proper synchronization to such signals.
Now, with this arrangement wherein plural adjacent tracks are recorded simultaneously, and wherein the signal supplied to the trailing head is delayed during recording while the signal reproduced by the leading head is delayed during playback, if the PCM audio signal is recorded in the after-recording mode, it is seen from FIGS. 4B and 4C that the trailing head is in position to play back the video signal while the leading head is in position to record the PCM audio signal. Similarly, the leading head is in position to play back the video signal while the trailing head is positioned to record the PCM audio signal. More particularly, at a time t.sub.0, the trailing head is positioned to begin recording the PCM audio signal, as shown in FIG. 4B, but because of the delay imparted to the signal reproduced by the leading head, at time t.sub.0 the video signal still is being reproduced therefrom. Conversely, at time t.sub.1, the recording of the delayed PCM audio signals supplied to the trailing head (as shown in FIG. 4B) ends and the head begins to reproduce the video signal. But, at this time, the PCM audio signal still is being played back by the leading head (as shown in FIG. 4C). It is not until time t.sub.2 that the delayed PCM audio signal reproduced by the leading head terminates. Hence, even though plural tracks are recorded simultaneously by plural heads, because of the aforementioned time delays needed during recording and reproduction to minimize crosstalk, the audio signal recorded in one track during the after-recording mode overlaps with the video signal reproduced from an adjacent track. Consequently, the aforementioned disadvantage of deterioration in the reproduced video signal due to this overlap is present.